One of the currently explored solutions for an efficient energy conversion, for electrical power generation and hydrogen production, is based on solid-state electrochemical devices such as solid oxide cells (SOC), operating at high temperatures, between 750 and 850 °C. To limit the chemical reactivity between the cell materials and mitigate the SOC degradation, one strategy consists in lowering the operation temperatures. This however also affects reaction kinetics of oxygen species and the charge transport properties, resulting in augmented polarization resistance of the cell, i.e. inferior electrochemical performance [1].

Recently, the study of oxygen electrode materials has focused on the Ruddlesden-Popper phase La₂NiO_4+δ (LNO), an oxygen-overstoichiometric mixed ionic-electronic conductor (MIEC) with relatively high oxygen diffusion and oxygen surface exchange coefficients compared to traditional perovskite materials [2]. Further optimization of the charge transfer of oxygen species was obtained by optimizing the microstructure of the oxygen electrode and the quality of interfaces between ionic and electronic conducting layers. The creation of nanostructured LNO active functional layers (AFL) using Electrostatic Spray Deposition (ESD) allowed for an important reduction of the polarization resistance [3]. However, detailed studies on the charge transfer mechanisms of oxygen species in LNO oxygen electrodes under polarization in a fuel cell (SOFC) mode highlighted the depletion of oxygen interstitials close to the interface between the AFL and the electrolyte. The consequence, limited electrode performance and reduced stability for prolonged SOFC operation, is a concern for the application of LNO-based SOC in the context of reversible energy conversion [4] [5].

We offer an innovative solution for improving the electrode performance by forming a composite of LNO with CGO (Ce_0.9Gd_0.1O_2-d), aiming to disperse the ionic conductor in the AFL while avoiding the chemical reactivity between the two components. In literature, the creation of LNO-CGO oxygen electrode composites was achieved using a conventional screen-printing technique [6]. In this work, the employment of the ESD technique allowed the realization of unique electrode architectures and particularly graded composites. The AFL deposits by ESD are characterized by a coral-type, hierarchical microstructure featuring a high degree of porosity which is important for the transport of gas in SOC application and provides a high specific surface area for the reactions. X-ray diffraction-assisted structural studies allowed for optimizing the chemical composition of the gradient composite and tailoring the interface between the ionic conducting phase and the MIEC. The investigation of electrochemical properties, using impedance spectroscopy, of selected architectures was performed on symmetrical cells, consisting of an electrolyte (Yttria-stabilized zirconia, YSZ), a CGO barrier layer, the AFL composites, and a current collecting layer (CCL) based on strontium-doped lanthanum manganite (LSM). Temperature-dependent acquisition of impedance spectra in synthetic air demonstrated an improvement of the polarization resistance for the gradient-type composite, compared to other architectures. Long-term stability tests and post-mortem analysis are ongoing.